The present invention relates to a device for fastening optical fibres.
The present invention relates especially to the application of fibre optical Bragg gratings, for use in sensors, where the fibre must often be glued for stability. A Bragg grating is a single modus fibre with a permanent periodic variation in the refraction index over a fibre length of e.g. 0,1 to 10 cm. The variation in the refraction index is established by illuminating the fibre with a UV-laser. A Bragg grating reflects light at a wavelength given by the refraction index and the space period of the refraction index variation (grating period), while light outside this wavelength is passing through the grating more or less unimpeded. The light reflected by the Bragg grating, will exhibit a wavelength which varies as a function of a measuring-size that changes the reflection index of the fibre material in the Bragg grating, and/or the length of the fibre in the grating zone (grating period). Tensions in the fibre or temperature will therefore give a change in the wavelength of the light that is reflected by the Bragg grating.
A number of devices exits for measuring stresses in mechanical constructions. In particular for application areas where little space is available, or there is high temperature, high tension etc., the known devices for measuring tension have disadvantages. For example, to measure tension underwater, tension sensors based on electrical elements are used, which in those kinds of environments have low reliability. In other cases, it may be that very little space is available for installing extra components, such as tension sensors based on induction or capacity (typical diameter is 10-20 mm). Another example is the monitoring of dams with sensors based on electrical strain gauges. In these cases it has happened that lightning has passivated the sensor element, and/or the electronics, and the monitoring of tension therefore does not happen.
Thus, a need has been identified for sensors with mainly passive components, which may be used in stringent environments and in narrow locations. Sensors based on optical fibres and Bragg technology may be developed for such applications, but this demands a fastening technique with the requirements for quality and size.
Examples of sensor designs with optical fibres and Bragg-technology, are, for example, given in the Norwegian patent applications 19973038, 19975657 and 19975656, in the name of the applicant, which were not published at the filing date of the present application. Examples of areas of application of these senors, are temperature monitoring of supply cables and pipelines, measuring of pressure in boreholes, and tensions in huge constructions or cables.
In practice, the fibre is glued to a sensor construction, in order to provide a stable fastening of devices which transfer tension to the grating as a function of, for example, pressure or tension. However, in certain applications, mechanical stress may be transferred to the optical fibre because of the different thermal expansion characteristics of the fastening device, and the fastening material, such as glue, which again might lead to variance in the results.
The object of the present invention is to provide a device for the fastening of optical fibres and/or Bragg gratings, which reduces the dangers of variance in the results caused by variations in the temperature, and offers simultaneously a method for producing very small sensors.
To achieve this and other objects, the invention relates to a device for the fastening of optical fibres in mechanical constructions, a device which will hereinafter also be termed a xe2x80x9csleevexe2x80x9d.
The object is fulfilled with a device according to the characterizing part of patent claim 1. Further advantageous features are given in the dependent claims.
The invention relates to a device for the fastening of optical fibres in mechanical constructions, the device will hereinafter also termed a xe2x80x9csleevexe2x80x9d.
The invention relates to a device for fastening an optical fibre with one or more Bragg gratings or equivalent, for use in a sensor means such as pressure, tension or bending sensors, wherein the optical fibre is tightly connected to a holding means by a fastening material such as adhesives based on polymers or metallic adhesives such as tin solder.
According to the invention, the holding device is provided with recesses or similar, in order to allow expansion of the fastening material in a direction generally radial to the length of the optical fibre.
The recesses may, for example, be established with, at the least, two longitudinal slits or similar, in the holding means (hereinafter also termed sleeve), in order to establish two cylinder-shell segments in the end of the sleeve. Upon installation of the optical fibre, a fastening agent, such as glue, is added in the cavity in the end of the sleeve in order to fasten the optical fibre to the holding means. With this embodiment of the holding means, the glue (and the segments in the end of the sleeve) will be allowed to expand and contract with variations in the temperature, with the assistance of these openings.
This fundamental embodiment of a sleeve, makes it possible to produce the device in a very small size, with automatic centring of the fibre and a natural cavity for the fastening agent. Furthermore, the device makes radial expansion of the fastening agent possible, but also offers a great longitudinal stability.
The holding means according to the invention, normally constitutes one of two fastening points for a fibre segment with an intervening Bragg grating.